Too dry for forest, too wet for desert
Why Are Grasslands Treeless?
An enormous belt of the world grows grass and almost no trees, and it takes three separate things to keep it that way.
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Step 1 of 5
Trees need deep, steady water.
A tree is a big investment that takes years to pay off, and it needs water all the way down to its roots for all of them. A few dry years kills a young tree outright.
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Step 2 of 5
Grass keeps its growing point underground.
A grass plant grows from its base, not its tip. Cut it, burn it or eat it and it simply grows back — which is fatal for anything that grows from the top.
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Step 3 of 5
Fire clears out anything woody.
Grassland burns easily and often. Grass regrows within weeks from its roots; young trees are simply killed, so fire keeps resetting the land to grass.
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Step 4 of 5
Grazing does the same job more slowly.
Huge herds crop everything back constantly. Grass tolerates it; tree seedlings are eaten before they get above knee height and never make it.
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Step 5 of 5
The soil underneath is superb.
Grass roots die and regrow constantly, packing the top metre with organic matter. That makes prairie soil some of the richest in the world — and it is nearly all farmland now.
Almost all of the world's original prairie has been ploughed. The soil that made it valuable is exactly what made it disappear.
The short version
Grasslands sit between forest and desert on rainfall, and fire and grazing finish the job of keeping trees out.
Try it yourself
Mow a lawn and watch it come back. Now imagine mowing a tree the same way.